Method and appliances for analog processing of a signal emitted by a particle detector

ABSTRACT

The invention concerns methods for processing a signal emitted by a particle detector. Said methods are characterised in that they consist in: detecting in said signal the portions where the signal is greater than a predetemined value V 1 , measuring the maximum value V max  reached by the signal in each of said portions, and associating with each of said portions an analog quantity which, at least in a predetermined range of values ΔV 1  of said maximum value V max , is an increasing function of (V max −V 1 ). The invention is applicable to various devices and appliances implementing said methods.

[0001] The present invention relates to the analysis of a flux ofparticles received by a particle detector during a given interval, forexample with the aim of counting these particles or measuring theirenergy. It relates more particularly to methods of analysis usinganalogue processing of the signal emitted by the said detector, as wellas the devices and apparatus intended for implementing these methods.

[0002] The detectors considered in the present invention are knowndetectors, whether of the point or matrix type, and regardless of thematerials, semiconducting or otherwise, of which they are composed. Thesignals emitted by these detectors can either be electric currents, orthey can be of a physical nature and can be converted to electriccurrent in a known manner. It will simply be assumed that reception of aparticle by the detector triggers an output signal having the form of apulse of a certain width and the maximum amplitude of which isrepresentative of the energy of this particle.

[0003] The invention applies to any field where the analysis of a fluxof particles, and in particular their counting or measurement of theirenergy, can be useful, for example in the case when these particles arephotons, in radiology, in fluoroscopy or in video imaging. It isparticularly suited to fields requiring a method of signal processingwhich, though of high quality (in the sense that the said method permitsvery accurate measurements of the flux), does this using a device ofsmall size; this is in particular the case when this device is composednot of a single detector (pixel), but of a matrix of pixels, because thesize of the electronic apparatus used is then limited by the spacing ofthe pixels.

[0004] In the rest of the description, for the purpose of clarity,reference will be made to the detection of “photons” (i.e. measurementof the characteristics of electromagnetic radiation), but it is to beborne in mind that the invention is completely independent of the natureof the particles detected.

[0005] An important factor limiting the quality of signal processing isthe background noise which is always present in the current emitted bythe detector. This background noise includes at least two components.The first component is the “dark current”, i.e. the fluctuating current,of thermal origin, emitted by the detector even when it is not receivingany photons. The second component is the “transient decay current”, i.e.the fluctuating current that is manifested for a certain time afterreception of a photon by the detector; in detectors using semiconductingmaterials, this transient decay current is due in particular to crystalimperfections in these materials.

[0006] Let us examine the effects of this background noise on theaccuracy of measurements carried out by the known methods.

[0007] If for example a measurement system based on integration is used,by means of which the total energy of the radiation received by thedetector during a predetermined time is measured, the current from thedetector is integrated over this time. A faithful representation of theenergy of the photons received then requires taking into account all ofthe current produced, including the low values: use of a threshold ofdetection of this current would therefore be detrimental, in that itwould cause a loss of information. However, the measured currentincludes, as explained above, a component due to the background noise,to which an exact value cannot be ascribed because of the fluctuationsdue to thermal drift and to the transient decay current, and because ofthe random noise associated with this component. In the known systems,the current due to the background noise is integrated in the measuredvalue, and then a quantity that is only an estimated average value ofthe effect of the background noise is subtracted, in order to obtain therepresentative value of the radiation energy.

[0008] As another example, when a measurement system based on countingis used, by means of which the number of photons of energy E above athreshold E₂ received by the detector during a predetermined time ismeasured, a suitable device (for example a bistable circuit) istriggered when the signal exceeds a certain threshold valuecorresponding to E₂, and the said device is reset when the signal fallsbelow this threshold value. Admittedly, there is then nothing to preventthe placement, at the detector output, of a system for filtering thecontinuous component from the background noise. However, the problem isthat it is not possible to distinguish an increase in current due to thearrival of a photon from the increase in current caused by a fluctuationof the background noise, unless the said threshold value is set highenough so that the fluctuations can practically never exceed it. Inaddition, in these conventional measurement systems, the bistablecircuits or similar devices produce parasitic coupling. In practice,this threshold cannot therefore be set very low.

[0009] A subject of the invention is therefore methods, and relativelycompact devices implementing these methods, which are intended to reducethe sensitivity of measurements of particle flux, on the one hand tofluctuations of the background noise present in the signals emitted bythe detectors, and on the other hand to disturbances caused by themeasurement electronics.

[0010] With this aim, the invention proposes a method of processing thesignal produced by a particle detector, the said method beingcharacterized in that

[0011] the portions where the signal is above a predetermined value V₁are detected in the said signal,

[0012] the maximum value V_(max) reached by the signal in each of thesaid portions is measured, and

[0013] an analogue quantity Q which, at least over a predetermined rangeof values ΔV₁ of the said maximum value V_(max), is a—for examplelinearly—increasing function of (V_(max)-V₁), is assigned to each of thesaid portions.

[0014] In fact, the invention exploits the fact that, in conventionaldetectors, the “peak” of each current pulse caused by an incidentparticle is proportional to the energy of that particle, or is at leastrepresentative of this energy (assuming for the purpose of clarity thatthe said pulse has positive values: the reader will easily transfer thecharacteristics of the invention to the case when negative values aremeasured). The method according to the invention thus only takes intoconsideration this peak (in the form of V_(max)), without taking intoaccount the rising part and the falling part of each pulse, and to aneven lesser extent the value of the signal between the pulses, so thatthe effect of fluctuations of the background noise is felt only for thebrief duration of these pulses, in the course of which the measurementsare carried out. This leads to an appreciable improvement of the qualityof the measurements relative to the conventional methods.

[0015] With regard to the practical choice of the detection thresholdV₁, it is clear that for this thresholding to be effective, generally avalue of V₁ must be chosen that is above the average level of thebackground noise (or must be chosen as positive if, at the detectoroutput, the continuous component is filtered from the background noise).This being so, the higher the value of V₁, the more the fluctuations ofthe background noise are avoided. However, the presence of thisthreshold V₁ prevents the detection of photons, the energy of which (ifapplicable) is lower than the energy E₁ associated with a voltage pulsepeaking at V₁; consequently, the higher the value of V₁, the larger willbe the energy band for which photons of energy belonging to this bandwill not be able to be detected. Therefore the value of V₁ must bechosen carefully, using these principles, as a function of theparticular application.

[0016] The variation, according to the invention, of the analoguequantity Q over the range of values ΔV₁ naturally makes it possible totake account of the energy variations of the photons received, and in away that can differ from one application of the invention to another. Inphotometry, for example, it will be possible to prefer a linearbehaviour of the analogue quantity Q as a function of the energy E ofthe photon. For photon counting, the increasing (but not necessarilylinear) function Q(E) means that it is possible to implement“progressive thresholding” about a predetermined energy, as explainedbelow.

[0017] According to particular characteristics of the invention, thereis assigned to each of the said portions an analogue quantity Q which isan increasing function, for example linear, of (V_(max)−V₁) if themaximum value V_(max) is below a second predetermined value V₂, andremains constant at its value for V_(max)=V₂ if the maximum valueV_(max) is above this second value V₂, at least over a predeterminedrange of values ΔV₂ of the said maximum value V_(max).

[0018] These particular characteristics are very advantageous when theinvention is applied to photon counting, which consists, it will berecalled, of measuring the number of photons with energy E above athreshold E₂ received by the detector during a predetermined time. Infact, an analogue quantity Q proportional to (E−E₁) will then beassigned to every photon of energy E between E₁ and E₂, and an analoguequantity Q₂ proportional to (E₂−E₁) will be assigned to every photon ofenergy E greater than E₂. In this way, progressive thresholding aboutE=E₂ is implemented.

[0019] It will be noted that this progressive thresholding has theadvantage, in comparison with very abrupt thresholding, of making itpossible to take account of peaks, the real energy of which is minimizedby a particularly low instantaneous level of background noise: in fact,the probability that a particle with apparent energy (as detected) belowthe counting threshold that has been adopted still has a real energyabove this threshold increases as the difference between this apparentenergy and the counting threshold decreases. This progressivethresholding can therefore be understood as the application, to theapparent energy of the observed peaks of a likelihood coefficient thatapproaches 1 as this apparent energy approaches E₂. In reality, thecounting threshold (above which it is desired to characterize theparticles) is below E₂ (but it is not necessary to know it) to theextent that there may also be peaks with apparent energy greater thantheir real energy because of a particularly high instantaneous level ofthe background noise: according to this approach, E₂ is the energy levelfor which it is considered that it is certain, regardless of theinstantaneous value of the background noise, that it is indeed aparticle that has a real energy at least equal to the above-mentionedcounting threshold.

[0020] Furthermore, this progressive thresholding makes it possible totake account of the fact that, in the analysis of a physical phenomenon,the energy transition between the significant particles and those thatare not, is not necessarily abrupt, and the particles with energy closeto the threshold can contribute to the phenomenon which it is beingattempted to characterize; progressive thresholding can then be analysedas assignment of a coefficient of efficiency of the peaks that movescloser to 1 as the energy level moves closer to E₂.

[0021] These two approaches can of course be combined, and the choice ofthe thresholds and of the slope of the rising portion makes it possibleto take best account of what it is desired to characterize; this choicecan be made, for example, following tests conducted in accurately knownconditions. It must be understood here that the linear form of such arising curve is particularly practical, especially because of the smallnumber of coefficients to be chosen, but that other forms are possible,in order to take best account of the results that are expected fromprogressive thresholding.

[0022] This progressive thresholding makes it possible to position thelevel above which the signal is measured, very close to the level of thebackground noise: in fact, a very low, but non-zero, likelihoodcoefficient is applied to the low peaks, taking account of theprobability that this low peak is representative not of a fluctuation ofthe background noise but of a particle that should be accounted for.

[0023] It can therefore be seen that, relative to the conventionalmethods, the invention offers the advantage of counting photons veryaccurately even when the energy E₂ is very low. An additional advantageis the absence of parasitic signals such as those produced, inconventional bistable-circuit counters, by the switchings of thisbistable circuit associated with incrementing of the counter.

[0024] According to even more particular characteristics of theinvention, there is assigned to each of the said portions an analoguequantity Q which

[0025] is an increasing function of (V_(max)−V₁) if the maximum valueV_(max) is below a second predetermined value V₂,

[0026] remains constant at its value for V_(max) =V₂ if the maximumvalue V_(max) is between this second value V₂ and a third predeterminedvalue V₃, and is a decreasing function of V_(max) if the maximum valueV_(max) is above this third value V₃, at least over a predeterminedrange of values ΔV₃ of the said maximum value V_(max).

[0027] Owing to these arrangements, it will be possible in particular toobtain a function Q(E) that is tooth-shaped, or approaches a Gaussiancurve. It will thus be possible to favour, in the radiation received,the photons belonging to a relatively narrow energy band, these photonsbeing particularly significant in the application envisaged.

[0028] According to another aspect, the invention relates to variousdevices.

[0029] Firstly, it thus relates to a device for processing the signalproduced by a particle detector, the said device comprising

[0030] a conversion unit that is able to convert any pulse of currentemitted from the said detector into a voltage pulse V,

[0031] an analogue circuit including

[0032] an electric charge storage device D₃,

[0033] a first electric charge receiver D₁ that can be fed by the saidcharge storage device D₃ in a manner controllable by means of the saidvoltage V, and

[0034] a second electric charge receiver D₂ that can also be fed by thesaid charge storage device D₃ in a manner controllable by means of thesaid voltage V, and

[0035] an apparatus for measuring the electric charge Q contained in thesaid second charge receiver D₂, the said analogue circuit being designedin such a manner that each voltage pulse V produces the followingeffects successively within the said device:

[0036] the said charge storage device D₃ is isolated from the said firstcharge receiver D₁,

[0037] the charge storage device D₃ is connected to the said secondcharge receiver D₂ when the voltage V exceeds a predetermined value V₁,

[0038] an electric charge Q that is an increasing function of (V−V₁)passes from D₃ to D₂,

[0039] the connection between D₃ and D₂ is cut when the voltage V beginsto decrease after reaching a maximum value V_(max), and

[0040] D₃ is reconnected to D₁ which restores the lost charge Q in D₃.

[0041] Secondly, the invention also relates to a device for processingthe signal produced by a particle detector, the said device comprising

[0042] a conversion unit that is able to convert any pulse of currentemitted from the said detector into a voltage pulse V,

[0043] an analogue circuit including

[0044] a charge storage device M₂,

[0045] a first electric charge receiver D₁ that can be fed by the saidcharge storage device M₂ in a manner controllable by means of the saidvoltage V, and

[0046] a second electric charge receiver D₂ that can also be fed by thesaid charge storage device M₂ in a manner controllable by means of thesaid voltage V, and

[0047] an apparatus for measuring the electric charge Q contained in thesaid second charge receiver D₂, the said analogue circuit being designedin such a manner that each voltage pulse V produces the followingeffects successively within the said device:

[0048] the said charge storage device M₂ is isolated from the said firstcharge receiver D₁,

[0049] the charge storage device M₂ is connected to the said secondcharge receiver D₂ when the voltage V exceeds a first predeterminedvalue V₁,

[0050] an electric charge Q proportional to (V−V₁) passes from M₂ to D₂if the voltage V does not exceed a second predetermined value V₂, orproportional to (V₂−V₁) if the voltage V exceeds the said second valueV₂,

[0051] the connection between M₂ and D₂ is cut when the voltage V beginsto decrease after reaching a maximum value V_(max), and

[0052] M₂ is reconnected to D₁ which restores the lost charge Q in M₂.

[0053] Thirdly, the invention also relates to a device for processingthe signal produced by a particle detector, the said device beingcharacterized in that it comprises two circuits similar to that brieflydescribed for the second device, and both receiving the voltage pulse Vemitted from a conversion unit, the parameters of these two circuitsbeing controlled independently of one another, and an analoguesubtractor that is able to produce an output signal equivalent to thedifference Q between the respective analogue charges Q′ and Q″transferred to the respective second charge receivers D′₂ and D″₂contained in the said circuits.

[0054] Finally, the invention fourthly relates to a device forprocessing the signals produced by a set of particle detectors, the saiddevice being characterized in that at least one of these signals isprocessed by means of a device such as those briefly described above.

[0055] For each of these devices, the measurements are carried out bysampling and reading this analogue quantity Q at predetermined points oftime. Since this quantity depends on V_(max), it is properlyrepresentative of the energy of the photon giving rise to the pulse. Inparticular, in order to count the photons received since the precedingmeasurement, it is sufficient to divide Q by Q₂.

[0056] The advantages offered by these devices are therefore essentiallythe same as those offered by the methods according to the invention, butit will be noted in addition that they can easily be constructed usingconventional semiconductor components, as will be shown in the detaileddescription given below, hence the small overall size of these devices,as well as a low cost of manufacture. These properties arise inparticular from the fact that, according to the invention, processing ofthe signal from the detector is purely analogue. Naturally, in certainapplications it will prove useful to connect an analogue-digitalconverter to a device according to the invention, in order to permitdigital processing of the information obtained, especially ifconstraints of cost and overall size are secondary in the applicationenvisaged.

[0057] Finally, the invention relates to various apparatus for analysinga flux of particles incorporating at least one device such as thosedescribed briefly above.

[0058] Other aspects and advantages of the invention will become evidenton reading the detailed description, given below, of particularembodiments given as non-limitative examples. This description refers tothe appended drawings, in which:

[0059]FIG. 1 schematically represents a device according to a firstembodiment of the invention,

[0060]FIGS. 2a to 2 d represent the main stages in the operation of thedevice shown in FIG. 1,

[0061]FIG. 3 schematically represents the relationship between theenergy E of an incident photon and the analogue quantity Q that isassociated with it, when using the device shown in FIG. 1,

[0062]FIG. 4 schematically represents a device according to a secondembodiment of the invention,

[0063]FIGS. 5 and 6 represent the main stages in the operation of thedevice shown in FIG. 4,

[0064]FIG. 7 schematically represents the relationship between theenergy E of an incident photon and the analogue quantity Q that isassociated with it, when using the device shown in FIG. 4,

[0065]FIG. 8 schematically represents a device according to a thirdembodiment of the invention, and

[0066]FIGS. 9a to 9 d schematically represent the relationship betweenthe energy E of an incident photon and the analogue quantity Q that isassociated with it, when using the device shown in FIG. 8 according tofour different settings, presented here as examples.

[0067]FIG. 1 represents, according to a first embodiment of theinvention, a device 100 that is intended for processing the signalsemitted by a photon detector 2.

[0068] This detector 2 emits, in response to the arrival of a photon onits receiving surface, a pulse of current I. According to the invention,firstly this pulse of current I is converted to a voltage pulse V withthe help of a suitable conventional unit 1.

[0069] The continuous component is then, optionally, removed from theresulting signal by means of a conventional filtering unit 5. It will berecalled that this continuous component corresponds to the average valueof the dark current and of the transient decay current leaving thedetector 2, whatever this detector.

[0070] The signal is then processed by the analogue circuit 3. Thevoltage pulse V is applied directly to a diffusion zone D₁ whichperforms the role here of an electric charge receiver, and to the gateof a MOS (metal oxide semiconductor) transistor M₃. More precisely, inthe embodiment represented, a transistor of the NMOS type was chosen forM₃, i.e. with conduction by electrons; the surface channel potential V*of M₃ is therefore lower here than V by a certain amount ε.

[0071] Between M₃ and D₁, there is another diffusion zone D₃, whichperforms the role of charge storage device, and another NMOS transistorM₁, the gate of which is maintained at a fixed potential V₁; the channelpotential V₁* of M₁ is less than V₁ by an amount close to ε.

[0072] Finally, after the transistor M₃, there is a final diffusion zoneD₂ which is intended to receive the analogue charge Q according to theinvention.

[0073] In order to carry out a measurement, this diffusion zone D₂ isbriefly brought to a predetermined fixed potential V_(R) (by closing andthen opening the switch S). The charge Q accumulated in D₂ then producesa voltage change that is read by a measuring apparatus 6 (for example acapacitor with voltmeter, or a ballistic galvanometer) supplying theoutput signal from the device V_(out).

[0074]FIG. 2 represents the main stages in the operation of the deviceshown in FIG. 1, showing schematically, for each stage, the relationshipbetween the potentials of D₁, D₂, D₃, and of the channels of M₁ and M₃,in the case when the value of V₁ is chosen in such a manner that thevalue of V₁*=V₁−ε is greater than the average value of the backgroundnoise, in the device 100 without the filtering unit 5 (or is positive,if such a unit 5 is incorporated).

[0075]FIG. 2a shows the values of these potentials in the absence of apulse from detector 2. It can be seen in particular that the chargeslocated in D₃ can flow into D₁, but not into D₂, because of thepotential barrier presented by the channel of M₃.

[0076] Following the reception of a photon by the detector (or becauseof a fluctuation of the background noise), the potentials of D₁ (i.e. V)and of M₃ (i.e. V*=V−ε) increase in concert. If the pulse is strongenough, the stage shown in FIG. 2b is reached, where communicationbetween D₃ and D₁ is cut.

[0077] If the pulse is strong enough, we then reach the stage shown inFIG. 2c, where the charges contained in D₃ can begin to flow into D₂.The quantity of charges thus moved for a given voltage V depends on theparasitic capacity of D₃.

[0078] When the pulse V reaches its maximum V_(max) (FIG. 2d), thecharge moved to D₂ has reached a certain value Q.

[0079] The voltage V then decreases, and M₃ immediately forms apotential barrier between D₃ and D₂, so that no additional charge flowsto D₂. Therefore the charge Q preserves the value which it acquired atthe peak of the pulse.

[0080] Finally, one returns to the situation in FIG. 2a until a newpulse arrives. It is necessary to ensure, taking into account thepractical frequency of arrival of the photons, that recharging of D₃from D₁ is fast enough for the device to be ready for this new pulse.

[0081]FIG. 3 shows the shape of the function Q(E) (where E is the energyof the incident photon that gave rise to the voltage pulse V) associatedwith the device 100. This curve Q(E) is characterized by a detectionthreshold E₁ corresponding to a voltage pulse, the peak V_(max) of whichis equal to the voltage V₁. An increasing portion can then be observed,at least over an energy band of the photons ΔE₁ corresponding to a rangeof values ΔV₁ of V over which the circuit 3 behaves faithfully in themanner described above.

[0082] In the case when it is necessary, for the application envisaged,to have a linear increase, it will be possible for example to replacethe diffusion zone D₃ with an NMOS transistor, the gate of which will bepolarized to a potential higher than the largest value expected forV_(max); or alternatively, it will be possible to connect the plate of acapacitor, the other plate of which is polarized to a fixed potential,to the diffusion zone D₃.

[0083]FIG. 4 represents, according to a second embodiment of theinvention, a device 200 that is intended for processing the signalsemitted by a photon detector 2.

[0084] This device 200 only differs from the device 100, and moreprecisely the circuit 7 only differs from circuit 3, in the replacementof diffusion zone D₃ with an NMOS transistor M₂, the gate of which isbrought to a fixed potential V₂.

[0085]FIG. 5 represents the main stages in the operation of the deviceshown in FIG. 4 for a photon, the energy E of which is below E₂, whereE₂ corresponds to a voltage pulse, the peak V_(max) of which is equal tothe voltage V₂.

[0086] The operation of the device in this case is completely analogousto the operation described with reference to FIG. 2. It is true that inthe present device, when E is greater than E₁ (the value thatcorresponds to a voltage pulse, the peak V_(max) of which is equal tothe voltage V₁), a stage is reached (starting from FIG. 5b) where acertain charge Q₂ is isolated in the channel of M₂, which was not thecase with the device in FIG. 1; but the value of this charge Q₂ has noeffect on the functioning of the present device if E is lower than E₂.

[0087] Therefore let us examine, with the help of FIG. 6, the mainstages in the operation of the device 200 for a photon, the energy E ofwhich is greater than the said value E₂.

[0088] The stages 6 a to 6 c are identical to the respective stages 5 ato 5 c. Then the charge Q₂ flows as previously from M₂ to D₂, but, whenV continues to increase, we reach a stage (V*>V₂*, where V₂* =V₂−ε, andtherefore V>V₂) where this charge is exhausted. Therefore when voltage Vreaches its maximum V_(max) (FIG. 6d), the charge deposited in D₂ isequal to Q₂ regardless of the value of this maximum (assumed to begreater than V₂).

[0089] The return to the initial state (FIG. 6a) is analogous to thereturn to the initial state in the previous devices.

[0090]FIG. 7 shows the form of the function Q(E) associated with thedevice 200.

[0091] This curve Q(E) is characterized by a detection threshold E₁,followed by a rising portion, the slope of which is determined by thecapacity of M₂. Then the function remains constant at a value Q₂=Q(E₂),at least over a photon energy band ΔE₂ corresponding to a range ofvalues ΔV₂ of V above V₂, over which the circuit 7 behaves faithfully inthe manner described above.

[0092]FIG. 8 represents, according to a third embodiment of theinvention, a device 300 that is intended for processing the signalsemitted by a photon detector 2.

[0093] This device 300 comprises, in addition to a current-to-voltageconversion unit 1 and (optionally) a filtering unit 5, two circuits 7′and 7″ that are functionally similar to the circuit 7 of the device 200.The charges Q′ and Q″ accumulated respectively on D′₂ and D″₂ produce,after measurement in units 6′ and 6″, respective output signals V′_(out)and V″_(out) which are sent to an analogue subtractor 4, so that theoutput signal from device 300 is V_(out)=V′_(out)−V″_(out).

[0094]FIGS. 9a to 9 d show the form of the function Q(E) (where Q isdefined here as being equal to (Q′−Q″)) associated with the device 300,for various values of V′₁, V′₂, Q′₂, V″₁, V″₂, and Q″₂.

[0095] In the case of FIG. 9a, a common value (called Q₀) is taken forQ′₂ and Q″₂, and equal capacities for M′₂ and M″₂ (so as to obtain equalslopes in the rising part of the functions Q′(E) and Q″(E)), and inaddition: V″₁=V′₂. We then obtain a triangular curve Q(E).

[0096] It may be desired to broaden the top of this curve, so that itbecomes more like a tooth, or a Gaussian curve. To do this (FIG. 9b), itis sufficient to take V″₁>V′₂.

[0097] By taking Q′₂ greater than Q″₂ (FIG. 9c), a strobe pulse isobtained which maintains a non-zero value of Q beyond E=E″₂.

[0098] Taking different capacities for M′₂ and M″₂ (FIG. 9d), asymmetricslopes for the rising part and the falling part of Q(E) are obtained.

[0099] On the basis of these few examples, a person skilled in the artwill easily be able to choose from among the numerous possible settingsof parameters so as to obtain the required function Q(E) according tothe application in question, among a large range of possible functionalforms.

[0100] Furthermore, it is self-evident that the devices shown in FIGS.1, 4 and 7 are deliberately simple examples of applications that areable to supply the functions Q(E) shown in FIGS. 3, 7 and 9respectively. In practice, a person skilled in the art will be able tomodify them by known techniques, so as to give them secondary advantagessuch as insensitivity to parasitic noise, a rate of charge transferthrough the device that is sufficiently fast, or stability of thecurrent sources, amplifiers or transformers used.

[0101] Moreover, for the purpose of clarity, it was assumed in the abovedescription that the voltage pulse at the output of the current/voltageconverter is positive. In the case of negative pulses, a person skilledin the art will have no difficulty in adapting the devices described,for example by replacing the NMOS transistors with PMOS transistors(with hole conduction).

[0102] The invention was described above referring to the analoguecharge Q accumulated on a detector which can be either a singledetector, or an individual pixel within a multipixel detector, i.e. madeup of a matrix or block of pixels.

[0103] In the case of a multipixel detector, there is certainly noreason why, if necessary, the analogue charges accumulated on several ofthese pixels should not be summed. This summation offers for example aparticular advantage in the case of counting, if it is assumed that theenergy of the photons to be counted is, as is often the case, within arelatively narrow band positioned slightly above a counting thresholdE₂. The present invention then makes it possible to correct the countingerrors that might result from the fact that a certain photon arrivesbetween two pixels (which gives rise, at the output of each pixel, tosignals I₁ and I₂, the sum of which is equal to the signal I that wouldhave been produced if the said photon had arrived inside a singlepixel).

[0104] This is because, if a conventional device is used, neither ofthese two signals I₁ and I₂ will be sufficient to trigger the counterassociated with the respective pixel, so that the said photon will notbe counted. Conversely, if a device according to the invention is used,an analogue quantity Q₁ proportional to I₁, and an analogue quantity Q₂proportional to I₂ will be recorded, so that the sum Q=Q₁+Q₂ will beroughly equal to Q₂, and this photon will be counted correctly.

[0105] It will be noted in conclusion that the present invention can beconsidered overall from a different point of view from that presented inthe introduction. In fact, the many examples presented in detail aboveillustrate the fact that the signal processing according to theinvention leads to an analogue charge Q which represents, in apredetermined manner, the energy E of the incident photons. In otherwords, the function Q(E) performs the role of a “weighting function” bymeans of which we can ascribe, if necessary, a different “weight” toeach photon according to its energy. It has also been shown, in the caseof the weighting functions presented, how they can be obtainedconcretely by means of devices using conventional analogue electroniccomponents. Taking inspiration from these examples, a person skilled inthe art will be able to elaborate a suitable device for obtainingessentially any desired weighting function depending on the applicationenvisaged, or even a device offering possibilities for adjustmentallowing various forms of weighting curves to be obtained, suitable fora range of applications envisaged.

1. Method of processing the signal emitted by a particle detector,characterized in that the portions where the signal is above apredetermined value V₁ are detected in the said signal, the maximumvalue V_(max) reached by the signal in each of the said portions ismeasured, and an analogue quantity Q which, at least over apredetermined range of values ΔV₁ of the said maximum value V_(max), isan increasing function of (V_(max)−V₁), is assigned to each of the saidportions.
 2. Method of signal processing according to claim 1,characterized in that the value of V₁ is at least equal to the averagelevel of the background noise present in the signal emitted by thedetector.
 3. Method of signal processing according to claim 1 or claim2, characterized in that, over the said range ΔV₁ of values of V_(max),the said analogue quantity Q is proportional to (V_(max)−V₁).
 4. Methodof signal processing according to claim 1 or claim 2, characterized inthat there is assigned to each of the said portions an analogue quantityQ which is an increasing function of (V_(max)−V₁) if the maximum valueV_(max) is below a second predetermined value V₂, and remains constantat its value for V_(max)=V₂ if the maximum value V_(max) is above thissecond value V₂, at least over a predetermined range of values ΔV₂ ofthe said maximum value V_(max).
 5. Method of signal processing accordingto claim 3 and claim 4, characterized in that there is assigned to eachof the said portions an analogue quantity Q which is proportional to(V_(max)−V₁) if the maximum value V_(max) is below a secondpredetermined value V₂, and remains constant at its value for V_(max)=V₂if the maximum value V_(max) is above this second value V₂, at leastover a predetermined range of values ΔV₂ of the said maximum valueV_(max).
 6. Method of signal processing according to claim 4,characterized in that there is assigned to each of the said portions ananalogue quantity Q which is an increasing function of (V_(max)−V₁) ifthe maximum value V_(max) is below a second predetermined value V₂,remains constant at its value for V_(max)=V₂ if the maximum valueV_(max) is between this second value V₂ and a third predetermined valueV₃, and is a decreasing function of V_(max) if the maximum value V_(max)is above this third value V₃, at least over a predetermined range ofvalues ΔV₃ of the said maximum value V_(max).
 7. Method of signalprocessing according to any one of the preceding claims, characterizedin that the said particles are photons.
 8. Device (100) for processingthe signal produced by a particle detector (2), comprising a conversionunit (1) that is able to convert any current pulse emitted from the saiddetector (2) into a voltage pulse V, an analogue circuit (3) comprisingan electric charge storage device D₃, a first electric charge receiverD₁ which can be fed by the said charge storage device D₃ in a mannercontrollable by means of the said voltage V, and a second electriccharge receiver D₂ which can also be fed by the said charge storagedevice D₃ in a manner controllable by means of the said voltage V, andan apparatus (6) for measuring the electric charge Q contained in thesaid second charge receiver D₂, the said analogue circuit (3) beingdesigned so that each voltage pulse V produces the following effectssuccessively within the said device: the said charge storage device D₃is isolated from the said first charge receiver D₁, the charge storagedevice D₃ is connected to the said second charge receiver D₂ when thevoltage V exceeds a predetermined value V₁, an electric charge Q that isan increasing function of (V−V₁) passes from D₃ to D₂, the connectionbetween D₃ and D₂ is cut when the voltage V begins to decrease afterreaching a maximum value V_(max), and D₃ is reconnected to D₁ whichrestores in D₃ the charge Q that was lost.
 9. Device (200) forprocessing the signal produced by a particle detector (2), comprising aconversion unit (1) that is able to convert any current pulse emittedfrom the said detector (2) into a voltage pulse V, an analogue circuit(7) comprising a charge storage device M₂, a first electric chargereceiver D₁ that can be fed by the said charge storage device M₂ in amanner controllable by means of the said voltage V, and a secondelectric charge receiver D₂ which can also be fed by the said chargestorage device M₂ in a manner controllable by means of the said voltageV, and an apparatus (6) for measuring the electric charge Q contained inthe said second charge receiver D₂, the said analogue circuit (7) beingdesigned so that each voltage pulse V produces the following effectssuccessively within the said device: the said charge storage device M₂is isolated from the said first charge receiver D₁, the charge storagedevice M₂ is connected to the said second charge receiver D₂ when thevoltage V exceeds a first predetermined value V₁, an electric charge Qproportional to (V−₁) if the voltage V does not exceed a secondpredetermined value V₂, or proportional to (V₂−V₁) if the voltage Vexceeds the said second value V₂, passes from M₂ to D₂, the connectionbetween M₂ and D₂ is cut when the voltage V begins to decrease afterreaching a maximum value V_(max), and M₂ is reconnected to D₁ whichrestores, in M₂, the charge Q that was lost.
 10. Device (300) forprocessing the signal produced by a particle detector (2), characterizedin that it comprises two circuits (7′, 7″) according to claim 9 bothreceiving the voltage pulse V emitted from a conversion unit (1), theparameters of these two circuits (7′, 7″) being controlled independentlyof one another, and an analogue subtractor (4) that is able to producean output signal equivalent to the difference Q between the respectiveanalogue charges Q′ and Q″ transferred to the respective second chargereceivers D′₂ and D″₂ contained in the said circuits (7′, 7″). 11.Device for processing the signals produced by a set of particledetectors, characterized in that at least one of these signals isprocessed by means of a device according to any one of claims 8 to 10.12. Signal processing device according to any one of the claims 8 to 11,characterized in that the said particles are photons.
 13. Radiologyapparatus, characterized in that it contains at least one deviceaccording to any one of claims 8 to
 11. 14. Video imaging apparatus,characterized in that it contains at least one device according to anyone of claims 8 to
 11. 15. Fluoroscopy apparatus, characterized in thatit contains at least one device according to any one of claims 8 to 11.